In technical fields associated with production of hot-rolled metal bars, improvements in yield and quality with an enlargement in the available size of products in accordance with endless finish rolling are highly valued.
In such endless hot rolling fields, a metal bar joining technique is important. The metal bar joining technique is a technique for joining the tail part of a leading hot rolling material and top part of a trailing hot rolling material (hereinafter, either referred to as a “metal bar”).
Joining of metal bars in a hot rolling process is achieved between a roughing mill and a finish rolling mill. When metal bars are joined subsequently after processing in the roughing mill, it is possible to endlessly roll the metal bars in a finish rolling process.
For endless finish rolling, accordingly, it is necessary to join metal bars traveling at high speed. For such joining, various techniques have been proposed.
Known techniques are classified into a melting joining method and a solid-phase joining method.
When metal bars are joined using the melting joining method, there is a drawback in that softening of the melted joint occurs because the temperature of the melted joint is higher than portions of the metal bars around the melted joint, so that the melted joint exhibits a degraded joining strength, as compared to the base metal of the metal bars.
For a technique known as the solid-state joining method, there is a metal bar joining method disclosed in Japanese Patent Unexamined Publication Heisei 9-17411 (hereinafter, referred to as “411 invention”).
The 411 invention is a technique for joining metal bars by overlapping tail part of a leading metal bar and top part of a trailing metal bar, simultaneously shearing the overlapped two metal bars, and joining sheared surfaces of the metal bars formed in the shearing process in which the sheared surfaces are brought into contact with each other.
When the 411 invention is applied to an endless hot rolling plant, there are many advantages in that joining can be simply achieved within a short time because a shearing process is used to achieve the joining, and it is possible to decrease a reduction in temperature occurring during a finish rolling process because the required space is small.
However, the 411 invention has a drawback because the shape of the formed joint is non-uniform, and inclusion of surface scales between the joined surfaces may occur, so a considerable degradation in the joining strength of the joint may occur.
Furthermore, when metal bars are joined using the 411 invention, there may be a drawback in that the joined surfaces may have unjoined portions, or weak joined portions, at upper and lower cross-sectional portions thereof and opposite lateral end portions thereof. In addition, the inclusion of surface scales between the joined surfaces may occur, so that a degradation in joining strength may occur.
Meanwhile, high carbon steel is a kind of steel useful for tools because it exhibits high strength at normal temperature.
Endless hot rolling is carried out for such high carbon steel, using a joining technique, since many advantages such as an increase in yield and thinness can be obtained.
In this case, however, there are many problems in performing the endless hot rolling because high carbon steel is very brittle.
When endless hot rolling is carried out for such high carbon steel, using a welding technique, it is necessary to perform pre-heating and post-heating treatments because various welding cracks such as high-temperature cracks or low-temperature cracks may be easily formed. For this reason, in association with high carbon steel, there are drawbacks of a low welding workability and increased welding task difficulty.
Also, where a melting joining method, such as an induced heating method, is used for high carbon steel, there is a problem in that scales remain as defects in a joint because the melting point of the scales is higher than that of the base metal, so that it is difficult to force out the scales toward a bead in accordance with a pressing process carried out after a heating process.
Furthermore, in the case of high carbon steel, carbon contained in a large amount in the base metal reacts with the atmosphere during the welding process, thereby producing CO or CO2 gas which is, in turn, present in the welding site in the form of air pores. The presence of such air pores causes the joint to exhibit a degradation in mechanical performance not only at normal temperature, but also at high temperature after the completion of the welding process.
Due to the above-mentioned problems, applications for endless hot rolling to high carbon steel have been made in few cases.
Therefore, the present invention has been made in view of the above-mentioned problems incurred in conventional cases, and it is an object of the invention to provide a shear-joining method enabling application of endless hot rolling to high carbon steel.
Another object of the invention is to establish a shear-joining condition capable of allowing a joint of metal bars of high carbon steel, which is formed when the metal bars are joined, to sufficiently bear a finish rolling load in a finish rolling process, and to secure tensile characteristics capable of allowing the joint to withstand a tensile force generated between finish rolling stands.